The invention concerns nonlinear impairments in a communications channel, and in a preferred embodiment, it is related to nonlinear impairments in a telephone line consisting of a pair of copper wires when used for digital subscriber line (DSL) communication. The invention may also relate to nonlinear impairments in other types of transmission lines consisting of metal conductors, including coaxial cables, whether these are used for DSL, or for other types of high speed or wideband communications.
DSL service is intended to extend and enhance the capabilities of the standard voice communication telephone network (generally referred to by the acronym POTS meaning “plain old telephone service”). Because of the worldwide scope of the installed telephone network it is an attractive candidate for shared use with the expanding high speed digital communication technology embodied in digital data transmission systems such as, the Integrated Digital Services Network, (IDSN), Asymmetric Digital Subscriber Line (ADSL), and Very high bit rate Digital Subscriber Line (VDSL). In this application we use the term DSL to refer to all types of digital communication technologies that operate on telephone or other conducting lines, including but not limited to those just mentioned. The voice channel of the standard telephone network utilizes only the relatively low frequency electromagnetic spectrum, and as the network's lines are actually capable of carrying broadband electromagnetic signals through the megahertz range, digital systems have been designed to share these lines without significant interference during simultaneous voice and data transmissions. Designed for transmission via the standard telephone network, the earliest digital system implementation having a transfer rate greater than 2400 bps was ISDN in 1984 with a basic rate of 144 Kbps. Continuing technical improvements and sophisticated modulation methods expanded the digital data transfer rates and, by 1999 ADSL systems installed in the standard telephone network were capable of transfer rates of up to 7 MHz. ADSL utilizes Discrete Multi-Tone (DMT) multi-carrier technology and QAM modulation for data transmission over the upper frequency band of the standard telephone network which it shares with telephone voice communications.
The standard telephone network has continued expanding in response to increasing national and international telephone system requirements. New segments of telephone line have been added to existing lines to create an inter-connected network providing broad geographical coverage. The latest telephone system technology provides high speed multiplexed fiber optic interconnections between central offices, but much of the standard telephone network consists of earlier installed twisted pairs of copper wire (also simply “twisted pairs”). These twisted pairs are particularly common in the connections between central telephone offices and the ultimate subscriber premises or other user sites.
In making the connection from the central office to a customer premises, a line will normally contain a number of splices. Splices between copper wires take many forms in the standard telephone system. In this application, a splice refers to the physical joining of the end of one section of wire with the beginning of another to retain them in contact in order to provide electrical continuity between them. Splices may be formed, for example, using solder joints, screw terminals, crimp-on connectors, punch-down blocks and twisted wires. Splices will commonly occur for example at a demarcation box where a line from a central office connects to in-house wiring of a customer premises. Businesses usually have punch-down blocks various internal lines are spliced to the appropriate external lines. Lines will commonly also be spliced at one or more junction boxes between the central office and customer premises, as well as at the central office itself.
It is well known that splices are susceptible to electrical impairment from corrosion or oxidation. Expansion and contraction due to thermal effects may also play an important role in degrading the quality of the electrical contact of a splice. These impairments may be nonlinear in nature. If uncorrected, such impairments cause attenuation and distortion of the electromagnetic signals traveling over a transmission line consisting of a twisted pair with one or more splices. These impairments are particularly burdensome for very high frequency and high amplitude digital transmission, limiting the transmission range to the subscriber site as well as the attainable error free data transfer rate. Of course, not all splices exhibit such deleterious effects on transmission. However, those that do are termed “bad” or “oxidized” splices in the rest of this discussion.
Although it is commonly known in the art that splices with poor or oxidized contacts could have a nonlinear character, it is not generally appreciated that a significant degree of nonlinearity is very frequently present in such splices. There are (at least) two mechanisms that produce such effects: (1) An oxide layer between the metal conductors where they contact each other in a splice produces a metal-oxide-metal (MOM) junction; (2) A very constricted metallic connection between the metal conductors where they contact each other in a splice produces a metal to metal point contact (MMPC) junction. Both of these mechanisms produce nonlinearities that are symmetrical in nature so that reversing the direction of current flow produces similar results. In MOM junctions the dominant conduction mechanism is quantum tunneling of electrons through the thin oxide layer. Analysis of the physics of tunneling shows that this junction has relatively poor conductivity near zero voltage, but that conductivity improves as the voltage increases. For the MMPC junction nonlinearity comes about when the diameter of the contact area between the two bulk conductors is small enough that it begins to approach the mean free path of the electrons in the metal. Analysis of this case shows an opposite type of nonlinearity, one in which the conductivity is relatively good near zero voltage but decreases as the voltage increases. Both types can be found in existing standard telephone network lines, but this invention pertains primarily to those nonlinear junctions of the MOM (or similar) type, which conduct poorly at low voltage.
It has long been appreciated that oxidized joints exist in lines of the standard telephone network, but two fortuitous operational aspects of the network tend to reduce interference caused by splices in telephone voice communication. In the first instance, when an “on hook” telephone is about to receive a call, the central office applies a ringing voltage of 90 v ac to the twisted pair connected to the phone, and an appreciable alternating current (ac) flows in the line. It has been conjectured that ringing current having a high enough amplitude breaks down the high resistance oxide film that may form in a splice in the twisted pair, allowing a partial ohmic low resistance contact between the wires at the splice. Second, the central telephone office provides a continuous direct current (dc) battery voltage, across the twisted pair connected to a subscriber's telephone. When the telephone is “on hook” it presents high impedance to the line, and no dc current flows from the central office battery through the subscriber's twisted pair. When the telephone is “off hook” for voice communication, the telephone's low impedance is across the twisted pair, and current flows from the central office battery, down the line and through the telephone set. Thus the ringing current and off hook current tend to reduce the deleterious effects of splice impairment on telephone communication by causing electrochemical changes to the splice. These changes are irreversible and remain in effect after the current is removed, i.e. when the phone stops ringing or is hung up. However, these effects are much less beneficial in the case of DSL data communications. This is because the signal levels are much higher in this case and therefore interact much more strongly with the bad splice. There is also a much greater sensitivity to any nonlinearity present in the splice characteristics.
In the art, telephone lines that do not have ring current or battery current—for example, ISDN lines—are termed ‘dry lines.’ In such lines it is a common practice to inject a current specifically for the purpose of reducing the severity of oxidized contacts in the line. Such a current is designated as “sealing current” or “wetting current”. Sealing current is intended to cause permanent change to the bad splices that remains when the current is removed, and also to discourage oxides from forming in the first place. The current may be applied continuously or applied periodically for a short duration. Some studies have suggested that sealing current may not be helpful. Sealing current is never applied to wet lines since they already benefit from the similar effects of ringing current and off hook current.